1. Field of the Invention
The present invention relates to an unmanned conveying device to be used for conveying a semiconductor wafer or a substrate for use in an electronic device material.
2. Prior Art
A semiconductor integrated circuit is normally manufactured in a clean room where air is always cleansed. As the integration of the semiconductor progresses, there occurs the problem in that a spontaneous oxide film is formed on the surface of the semiconductor wafer or the substrate of the electronic device material. It is necessary to restrain the spontaneous oxide film from growing in view of improving the manufacturing yield.
FIGS. 14 and 15 are graphs disclosed in the "Super LSI ultra clean technology symposium" (Nov. 19, 1990), in which FIG. 14 shows a relation between a spontaneous oxide film thickness and time involved in forming the spontaneous oxide film on the surface of a silicon semiconductor wafer (Formed by oxygen and moisture in air) and FIG. 15 shows a relation between resistivity and time involved in forming the spontaneous oxide film. As evident from FIG. 14, when the silicon semiconductor wafer is exposed to the atmosphere, the growth rate of the oxide film is increased after the lapse of 100 to 200 minutes and the resistivity is sharply increased after the lapse of about one hour as illustrated in FIG. 15.
In the clean room, an unmanned carriage having a loading robot mounted thereon is used for conveying the semiconductor wafer from one spot where one manufacturing step is carried out to another spot where another manufacturing step is carried out. The time involved in the conveyance between two spots is normally relatively short time, e.g. several minutes. Since it takes more than one hour for the spontaneous oxide film to be formed on the surface of the semiconductor wafer, the formation of the spontaneous oxide film can be ignored during the conveyance so that the conveyance between the manufacturing spots has been made in the atmosphere.
It is a matter of course that the formation of the spontaneous oxide film can be ignored when the conveying system employing the unmanned carriage is normally operated. However, when the unmanned carriage does not operate as scheduled due to accidents such as a breakdown of a destination semiconductor manufacturing apparatus to which the unmanned carriage reaches, a breakdown of the conveying system, a stoppage of appliance and apparatus because of the power failure and an obstruction in the conveying route, the semiconductor wafer is left alone on the conveying carriage for a long time, which caused such a problem that the formation of the spontaneous oxide film grows rapidly during the period.
It is a first object of the present invention to provide an unmanned conveying device capable of restraining the spontaneous oxide film from growing in case that the conveyance is obliged to be interrupted for a long time due to its own cause or the cause of the destination spot, etc.
It is a second object of the present invention to provide an unmanned conveying device for a clean room with high safety, which is capable of restraining the spontaneous oxide film from growing without making the carriage largescale or heavyweight by obtaining inert gas from an inert gas reserving source which is installed on the ground.